In electrophotographic copying machines based on Carlson's method, charging the surface of a photoreceptor is followed by exposure to form an electrostatic latent image, which is developed with a toner, and the resulting visible image is transferred and fixed onto a transferee such as paper. At the same time, the photoreceptor is treated to remove the adhering toner, eliminate the charge and clean the surface to ensure long-term repeated use.
Therefore, the electrophotographic photoreceptor is required to have good physical properties such as printability, wear resistance and moisture resistance after repeated use and resistance to ozone generated upon corona discharge and to ultraviolet rays generated upon exposure (environmental resistance), as well as charging property and electrophotographic properties such as high sensitivity and low dark decay.
Conventional electrophotographic photoreceptors in common use are inorganic photoreceptors having a light-sensitive layer based mainly on an inorganic photoelectroconductive material such as selenium, zinc oxide or cadmium sulfide.
Meantime, the use of various organic photoelectroconductive materials for the light-sensitive layer of electrophotographic photoreceptor has recently been the subject of active R & D activities.
With respect to property requirements, the same applies to organic photoelectroconductive photoreceptors as for inorganic photoreceptors . For example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 71856/1988 and 71857/1988 propose the addition of particular compounds to improve performance.
However, improvements in potential stability and photoreceptor durability after repeated copying still remain unsatisfactory, requiring further improvement.
For example, Japanese Patent O.P.I. Publication Nos. 71855/1988, 71857/1988, 52150/1988, 50848/1988, 50849/1988, 50850/1988 and 50851/1988 propose the use of some additives for improving durability. These proposals aim mainly at prevention of photoreceptor performance degradation due to ozone generated upon charging. It should be noted, however, that photoreceptor durability degradation is caused not only by ozonic oxidation but also by repeated migration of electrons and holes in the light-sensitive layer and by NOx and SOx; in addition to ozonic oxidation, chargeability decreases and residual potential rises in repeated use.
A means or additive solving all these problems is demanded.